The invention relates generally to semiconductor devices and integrated circuit fabrication and, in particular, to fabrication methods and device structures for bipolar junction transistors and heterojunction bipolar transistors.
Bipolar junction transistors may be found, among other end uses, in RF transceivers, multi-gigabit analog-to-digital converters, optical networks, automotive radar, and high-speed circuits. Bipolar junction transistors may be combined with complementary metal-oxide-semiconductor (CMOS) field effect transistors in bipolar complementary metal-oxide-semiconductor (BiCMOS) integrated circuits, which take advantage of the favorable characteristics of both transistor types.
Bipolar junction transistors are three-terminal electronic devices that include an emitter, an intrinsic base, and a collector defined by regions of different semiconductor materials. In the device structure, the intrinsic base situated between the emitter and collector. An NPN bipolar junction transistor may include n-type semiconductor material regions constituting the emitter and collector, and a region of p-type semiconductor material constituting the intrinsic base. A PNP bipolar junction transistor includes p-type semiconductor material regions constituting the emitter and collector, and a region of n-type semiconductor material constituting the intrinsic base. In operation, the base-emitter junction is forward biased and the base-collector junction is reverse biased. The collector-emitter current may be controlled by the base-emitter voltage.
A heterojunction bipolar transistor is a variety of bipolar junction transistor in which two or more of the terminals-emitter, intrinsic base, and/or collector—are composed of semiconductor materials with unequal band gaps, which creates heterojunctions. For example, the base of a heterojunction bipolar transistor may be composed of silicon germanium (SiGe), which is characterized by a narrower band gap than silicon.
Device performance may be improved by thinning the layers of semiconductor material used to form the terminals, optimizing the germanium concentration profile across the thickness of the layer forming the base, adding carbon to the semiconductor layer forming the base in order to reduce the diffusion of the electrically-active dopant, and reducing the number of thermal cycles to minimize diffusion. Despite these measures, improved fabrication methods and device structures are needed for bipolar junction transistors and heterojunction bipolar transistors.